How to perform actions on a variable in a foreach while being able to use it afterwards - rust

Question

I have the following example code (including use statements too so there's some context on types):
use actix_web::{HttpRequest, HttpResponse, Responder};
use awc::ClientRequest;
pub async fn proxy(req: HttpRequest) -> impl Responder {
let response = construct_request(req).send().await;
let body = response.unwrap().body().await.expect("");
HttpResponse::Ok().body(body)
}
fn construct_request(req: HttpRequest) -> ClientRequest {
let client = awc::Client::default();
let mut new_req = client.get("url");
req.headers().iter().for_each(|headerPair| {
new_req.append_header((headerPair.0, headerPair.1));
});
new_req.content_type("application/json")
}
Currently, the compiler complains that I'm using the new_req value after it being moved in the foreach loop to copy the headers over, which is fair enough, that is what's happening.
error[E0507]: cannot move out of `new_req`, a captured variable in an `FnMut` closure
--> src/routes/proxy.rs:18:9
|
15 | let mut new_req = client.get("http://localhost:8001/ping");
| ----------- captured outer variable
16 |
17 | req.headers().iter().for_each(|headerPair| {
| ___________________________________-
18 | | new_req.append_header((headerPair.0, headerPair.1));
| | ^^^^^^^ ------------------------------------------- `new_req` moved due to this method call
| | |
| | move occurs because `new_req` has type `ClientRequest`, which does not implement the `Copy` trait
19 | | });
| |_____- captured by this `FnMut` closure
|
note: this function takes ownership of the receiver `self`, which moves `new_req`
--> /Users/{sanitized}/awc-3.0.1/src/request.rs:186:30
|
186 | pub fn append_header(mut self, header: impl TryIntoHeaderPair) -> Self {
| ^^^^
warning: variable does not need to be mutable
--> src/routes/proxy.rs:15:9
|
15 | let mut new_req = client.get("http://localhost:8001/ping");
| ----^^^^^^^
| |
| help: remove this `mut`
|
= note: `#[warn(unused_mut)]` on by default
error[E0382]: use of moved value: `new_req`
--> src/routes/proxy.rs:21:5
|
15 | let mut new_req = client.get("http://localhost:8001/ping");
| ----------- move occurs because `new_req` has type `ClientRequest`, which does not implement the `Copy` trait
16 |
17 | req.headers().iter().for_each(|headerPair| {
| ------------ value moved into closure here
18 | new_req.append_header((headerPair.0, headerPair.1));
| ------- variable moved due to use in closure
...
21 | new_req.append_header(("custom", req.headers().get("custom").unwrap()))
| ^^^^^^^ value used here after move
What I can't find out is how I can work around this so that I can continue to use new_req after the foreach loop - is this possible? I've tried doing some googling around preventing this movement but I haven't managed to find anything (I'm assuming this is a very simple resolution that I just don't have the right words to discover)

Answer 1

You'll find by looking at the signature of .append_header() and other methods that they will consume self and return Self. This is a type of builder-pattern that is designed to work like this:
let new_req = client.get("url")
.append_header(...)
.append_header(...)
.append_header(...)
.content_type("application/json");
or like this:
let mut new_req = client.get("url");
new_req = new_req.append_header(...);
new_req = new_req.append_header(...);
new_req = new_req.append_header(...);
new_req = new_req.content_type("application/json");
Unfortunately, constant ownership transfers like this don't play well with closures if you want to keep the value afterwards. You're probably better suited just using a for-loop instead:
for header_pair in req.headers() {
new_req = new_req.append_header(header_pair);
}
Or if you insist on using .for_each(), you can modify the request in-place by using .headers_mut():
req.headers().iter().for_each(|header_pair| {
new_req.headers_mut().insert(header_pair.0.clone(), header_pair.1.clone());
});
If you're in the uncomfortable situation where you must take and reassign ownership and it must be while captured in a closure, you'll have to employ a trick using Option:
// put it in an option
let mut new_req_opt = Some(new_req);
req.headers().iter().for_each(|header_pair| {
// take it out of the option
let mut new_req = new_req_opt.take().unwrap();
// do your thing
new_req = new_req.append_header(header_pair);
// put it back into the option
new_req_opt = Some(new_req);
});
// take it out again at the end
let new_req = new_req_opt.unwrap();